The present invention relates to data communications systems, particularly the transmission of data over voiceband telephone channels using modems.
Today, information and data services are provided to both the business and residential sectors through the use of modems in the telephone network. Over the years, the amount of information and the complexity of data services has increased and, as a result, modem speeds have also increased. For example, currently, modem speeds over the telephone network are as high as 28.8 kilobit per second. Indeed, continued growth in information and data services will require that modem speeds continue to increase.
However, in order to support high speeds, a modem uses a dense constellation of signal points to transmit data through the telephone network from a transmitting modem to a receiving modem. Unfortunately, any telecommunications system (like the transmitting modem, telephone network and the receiving modem) introduces distortion that is proportional to the transmitted signal level. Sources of such distortion include quantization noise, adaptive differential pulse code modulation, and inter-modulation distortion. This amount of distortion is proportional to the transmitted signal power, which itself is proportional to the distance of a signal point from the center of the signal point constellation. In other words, the signal points that are further away from the origin of the constellation, i.e., that have more transmitted signal power, are the most affected by any distortion when finally received by the receiving modem. Consequently, the amount of distortion that can be tolerated in a communications system may limit the data rate, or speed, of the system.
In order to reduce the effect of distortion on transmitted data signals, those in the art have designed, with particularity, the transmitter and receiver components of the communications system. For example, the co-pending, commonly assigned, United States patent application of W. Betts et al. entitled "Non-linear Encoder and Decoder for Information Transmission Through Non-linear Channels," Ser. No. 07/754,107, filed on Sept. 3, 1991, discloses a technique for changing the shape of the transmitted signal point constellation prior to transmission of any signal point to a receiving modem. In particular, the transmitted signal point constellation is "warped" inversely to the expected distortion. As a result, the receiving modem must compensate for the altered shape of the signal point constellation in processing any received signal point. Since the transmitter and receiver are, in a sense, "matched," the effect of distortion on a transmitted data signal is reduced.